The present invention relates to an apparatus for simultaneously detecting several ranges of wavelengths of polychromatic light radiation, and in particular to a multi-channel spectrometer with spectral background correction.
Conventionally, light intensity is determined at different locations of the spectrum using a polychromator. If design constraints require an especially compact photometer, or if especially high light fluxes must pass through the measurement device, then as a rule interference filters are used in order to filter out the spectral range of interest.
To make possible quantitative measurements, the measurement signal must be corrected for the spectral background. One known method is to install an additional interference filter which selectively eliminates the background radiation. However, this method requires that the light being examined be evenly split onto different filters. Inaccuracies in dividing the light beam may result to varying degrees in mistakes in correcting the background of a signal. Moreover, each measurement channel requires its own interference filter, so that when high-resolution interference filters must be used, the cost of such systems increases correspondingly.
One alternative is provided by a system with dynamic background correction. Such a system includes an interference filter capable of being tilted or tipped from which the light exits at a specific central wavelength depending on the angle of incidence. Due to the design of interference filters, as the angle of incidence increases, there is a shift of the central wavelength toward shorter wavelengths. The change of the central wavelength can be determined as a function of the angle of incidence by the formula below ##EQU1## where .lambda.=central wavelength for angle of incidence .alpha.;
.lambda..sub.O =central wavelength for normal incidence; PA1 n.sub.e =effective index of refraction of the filter; PA1 .alpha.=angle of incidence of the light.
This relation applies to collimated light at an angle of incidence less than 10.degree.. When this value is exceeded, the shift of the wavelength entails a distortion of the passband of the interference filter and a lowering of the transmission.
E. Cammann et al. describe in Fresenius Z. Anal. Chem. 1988, 331:336-341 a system of element-selective plasma-emission detectors for gas-chromatographical analysis. Dissociated compounds are pyrolytically decomposed into their elements in an energy-rich inert or noble gas plasma. The mixture so created is energized and the subsequently emitted light is fed by means of optic fibers to the element-specific detectors. An interference filter is provided for each detector and is tipped to and fro by 10 to 15 degrees at a rate of about 20/s. The detector determines the intensity of the measured and background radiation over a given element-specific range of wavelengths.
However, such systems are disadvantageous in that their mechanical components are exceedingly complex. With rapidly varying detected signals, the tipping rate must be very high to allow simultaneous determination of the measured signal and background. Rates beyond 20 Hz are technically very difficult to achieve, and therefore ultimately will be limited.